REPRODUCTIVE ELEMENTS IN APUS AND BRANGHIPUS. 279 



egg formation in Apus, and are seen in all stages bulging out 

 the membrane of the gland into the body-cavity. But it is by 

 no means the rule that these tetrad groups are formed in this 

 manner from one cell. In some cases two nuclei in adjacent 

 cells divide, and the derivatives nearest the periphery divide 

 again to form the group (fig. 25), or the group formation may 

 proceed in a much more irregular fashion out of one or two 

 secondary divisions of adjacent cells (fig. 27). 



The dividing chromosomes within the nuclei at times present 

 the very curious appearance represented in figs. 34, 35. It will 

 be here seen, that between the separating, more darkly stained 

 portions stretches a stainless band, which again suggests that 

 the stain only affects particles suspended in a clear fluid, and 

 that this fluid is non-miscible. Moreover it would seem that 

 these particles tend to run together into chromatic drops, 

 leaving a clear fluid (paranuclein). 



The initial impulse, whatever it may be, which gives rise to 

 the groups of four chromosomes, and is ultimately concerned 

 in the formation of the egg, is seen also to afi'ect the surround- 

 ing nuclei, which divide in the same peculiar manner again and 

 again, until they form a narrow stalk connecting the original 

 group of four with the cavity of the gland (fig. 42). The 

 extreme minuteness to which this subdivision is carried will 

 be seen (fig. 33), where it will be observed that all trace of 

 cell membrane is fast disappearing. The minute, free, nuclear 

 elements then left spread over the surface of the tetrad group 

 as a thin protoplasmic membrane, in which they rest without 

 dividing walls of any kind (fig. 42, a). 



During all this multiplication of the nuclei, the character of 

 their division remains precisely the same. In every division 

 the single chromatic ball passes through the metamorphosis 

 represented in figs. 36 — 41, the nuclear membrane contracting 

 until two precisely similar nuclei are left in the place of one. 



It will be obvious that this method of procedure, though on 

 the face of it approaching akinetic or direct division, is in 

 reality very diff'erent from the process as it appears in Branchi- 

 pus, or in the other forms in which it has been described. In 



